The present invention relates generally to an enhanced location methodology for a location system, and is especially applicable to a timing-based location system.
Many modem location systems, such as Loran and the Global Positioning System, (GPS), are timing based. Measurements of the propagation delay of a signal between the target device (for which the location is to be determined) and one or more network devices (for which the locations are known) are used to obtain distance information, assuming that the time a signal takes to propagate is proportional to the distance the signal travels. Ranging systems estimate the distance between the target device and a network device, and time difference of arrival (TDOA) systems estimate the differences in the distances between the target device and a plurality of network devices based on the differences in the arrival times of signals traveling between the target device and the network devices. In both cases, these distance estimates are used to estimate the location of the target device.
The simplest form of such a system is ranging. In a ranging system, a synchronization signal is sent from a network device to a target device, and the target device, in response, sends a timing signal that is detected by the network device. The network device is generally in a known location. The total propagation delay, multiplied by the propagation speed of the signal, gives the total distance traveled by the synchronization signal and the timing signal, which is equal to twice the distance between the target device and the network device (range data). This sequence may be reversed, with the target device sending the synchronization signal and the network device sending the timing signal, if the delay measurement is to take place on the target device.
The distance between the target device and the network device, in two dimensions, defines a circular locus of points on which the device is located. This distance is generally combined with other information, such as the angle of arrival of the timing signal, to determine a location of the target device relative to the network device. In some implementations, multiple network devices are used to determine multiple distances between the target device and multiple known locations of network devices. However, using multiple ranging devices has the disadvantage of requiring the sending of a synchronization signal and a timing signal for each network device for which this ranging information is to be measured. The channel loading requirements are then the product of the number of target devices supported and the number of network devices involved with each target device. This second-order loading requirement often limits the number of location measurements that may be made in a given time frame.
In TDOA systems, such as Loran and GPS, a target device may send an unsynchronized signal that is detected by several network devices that are synchronized with each other, or a target device may receive signals from several network devices that are synchronized with each other but not synchronized with the target device. Although this does not directly give range data, it does give information about the differences in the distances between the target device and the various network devices (pseudo-range data). For any two network devices, the difference in the two propagation delays represents the difference in the distances from the target device to each of the network devices. As is well known in the art of location systems, this difference between the distances from the target device to each of two network devices establishes the set of possible locations as (in two dimensions) a hyperbolic locus. If three or more properly placed network devices (i.e., three devices not on the same line) are used in the measurement, it is usually possible to arrive at a unique two-dimensional solution (although certain cases may require a fourth properly placed network device). Most TDOA techniques involve heuristic or iterative solutions, since it is difficult to solve for the intersection of multiple arbitrary hyperbolae in a plane. Extension to three dimensions, requiring finding the intersection of multiple arbitrary hyperboloids, will only exacerbate this difficulty.
Both of these techniques described above suffer from an effect known in the art of location systems as geometric dilution of precision, or GDOP. A primary cause of this phenomenon is the failure of the circular or hyperbolic solution loci to intersect at right angles. Any timing error in the arrival time measurements or synchronizations will be multiplied by the cosecant of the angle of intersection of the two loci. The value of the cosecant is one (1) in the rare case where the angle of intersection is 90 degrees, and is greater at other angles, approaching infinity when the angle of intersection approaches zero or 180 degrees. For this reason, the three dimensional performance of GPS is limited on the vertical (height) axis because visible satellites are restricted to one side of the target device on the vertical axis, resulting in poor GDOP. The horizontal accuracy of the three dimensional performance of GPS is limited in occluded situations where part of the sky is not visible to the target device for the same reason. Further, most terrestrial based TDOA systems, in which the network devices are located substantially in a plane, are substantially incapable of offering any accurate vertical location information without supplementing the timing information with less reliable data, such as angle of arrival. Often, ranging is not an option in these systems due to the potential channel loading considerations.
Thus, there exists a need for a technique which can offer improved range estimates without imposing a second-order channel loading requirement, which can provide a simple closed-form location solution instead of a complex or iterative algorithm, which can offer vertical location solutions using network devices arranged on a horizontal plane, and which can offer solution loci which intersect at angles as close as possible to 90 degrees.